Most aircraft engines finding use in military applications, such as air combat, reconnaissance and surveillance, are augmented turbofans. Augmentation provides additional thrust for the aircraft when called upon, that is, on-demand.
All turbofan engines include at least two air streams. All air utilized by the engine initially passes through a fan, and then it is split into the two air streams. The inner air stream is referred to as core air and passes into the compressor portion of the engine, where it is compressed. This air then is fed to the combustor portion of the engine where it is mixed with fuel and the fuel is combusted. The combustion gases then are expanded through the turbine portion of the engine, which extracts energy from the hot combustion gases, the extracted energy being used to run the compressor and the fan and to produce electrical power to operate accessories. The remaining hot gases then flow into the exhaust portion of the engine, producing the thrust that provides forward motion to the aircraft.
The outer air flow stream bypasses the engine core and is pressurized by the fan. No other work is done on the outer air flow stream which continues axially down the engine but outside the core. The bypass air flow stream also can be used to accomplish aircraft cooling by the introduction of heat exchangers in the fan stream. Downstream of the turbine, the outer air flow stream is used to cool the exhaust system. When additional thrust is required (demanded), some of the fan bypass air flow stream is redirected to the augmenter where it is mixed with core flow and fuel to provide the additional thrust to move the aircraft.
At the rear of the exhaust, a convergent-divergent (C-D) nozzle sets the correct back pressure so that the core runs optimally. The C-D nozzle accomplishes this by choking the gas flow through the nozzle throat, A8, and varying A8 as required to set the required mass flow.
Certain variable cycle aircraft engines achieve relatively constant airflow as thrust is varied by changing the amount of fan bypass flow utilizing a third duct. Aircraft utilizing these variable cycle engines are able to maintain inlet airflow at subsonic power settings more efficiently and over a broader flight envelope. One particular type of variable cycle engine is referred to as a FLADE™ engine, FLADE™ being an acronym for “blade-on-fan” and is characterized by an outer fan duct which flows air into a third air duct, the outer fan duct being generally co-annular with, and circumscribing the inner fan duct, which in turn, is co-annular and circumscribes the core. This third airstream is pressurized by a blade-on-fan arrangement as set forth in prior art FLADE™ disclosures. The FLADE™ blades are radially outward of and directly connected to rotating fan blades, the fan blades assembled to a disk mounted on a shaft. The position of the FLADE™ is a design consideration, the design selected based on the temperature and pressure of the FLADE™ air (third stream air) desired. The trade-off is based on the fact that a higher pressure of FLADE™ operating air produces FLADE™ operating air with a higher temperature. U.S. Pat. No. 5,404,713 issued to Johnson on Apr. 11, 1995, assigned to the Assignee of the present invention and incorporated herein by reference.
In these variable cycle designs, the inlet air can be split to form a third stream of air, which is in addition to the bypass and core. This third stream of air may be provided at a lower temperature and pressure than either the core air stream or the bypass air stream discussed above. The pressure of this third stream of air can be increased, while still maintaining it at a temperature and pressure below the bypass air stream, using the blade-on-fan or FLADE™ airfoil and duct. Prior art third stream air flows have been exhausted into the core exhaust either just fore or aft of the C-D nozzle. However, placement of heat exchangers within the third air stream in recent embodiments to take advantage of the low temperatures of the air flowing in the third stream duct or FLADE™ duct have resulted in pressure drops of the air in the third stream duct or FLADE™ duct. The changes in pressure by the introduction of heat exchangers have resulted in the inability to exhaust the third stream air into the core exhaust at conditions in which exhaust pressure is high, such as at high power operation, and the inlet pressure to the third stream is low, such as low Mach points. The result would be cessation of flow of air, or insufficient flow of air, in the third stream duct under these flight conditions, which could result in stagnation of air flow in the third stream duct and even backflow of gases (reversal of flow). Stagnation of the third stream flow air can lead to stall conditions on the blade-on-fan arrangement under certain circumstances, resulting in possible hardware damage and additional drag on the aircraft due to fan inlet spill drag.
What is needed is an arrangement in which the third stream duct air can be exhausted continuously so that there is no cessation or significant reduction of air flow in the third stream duct or in the FLADE™ duct at any operational conditions of the engine, as insufficient air flow could adversely affect cooling of heat exchangers or other hardware dependent on third stream air for cooling. Ideally, the third stream duct air flow should be exhausted to a low pressure region in a manner that will add thrust and operability to the aircraft.